Scan velocity modulation is known in beam deflection of cathode ray tubes, and is employed to enhance the perceived sharpness of the displayed picture. The basic concept of scan velocity modulation is the modulation of the horizontal scanning velocity of the electron beam at a light to dark or dark to light image transitions. For example, when a scan crosses a bright vertical bar on a dark background, the objective is a display where the screen brightness rises instantly to a maximum as the horizontal scan crosses the bright vertical bar and then drops instantly to the minimum level after passing the bar. However, the rise and fall times of the electron beam current are not instantaneous. In a display without scan velocity modulation, there are shades of gray at the transitions which blur the edges and are perceived as a softening, or lack of sharpness.
With beam scan velocity modulation, circuitry is provided to anticipate a transition in the luminance signal, and to modify the horizontal scanning speed such that the beam is accelerated at the dark area adjacent the transition, i.e., scanned at a rate in excess of the average scan rate. The time gained by this acceleration of the beam is used on the bright side of the transition, where the beam is decelerated to below the average scanning rate, thus the total time taken to scan horizontal lines remains constant, with the acceleration and deceleration canceling one another. The effect of increasing the scanning beam velocity in the dark area prior to the bright transition results in less beam current hitting the phosphor and consequently less phosphor excitation which makes the dark area darker. The slowed scanning velocity in the bright area causes the beam to excite the phosphors for a slightly longer period and results in an additional brightening immediately after the transition. The overall effect of the velocity modulation at an edge transition is to cause the transition to appear sharper than if scanned at a constant rate. At transitions from a light background to a dark area the process is reversed. The net effect is to increase the perceived sharpness of image edges, and thus the picture appears to be sharper and of greater resolution.
The beam velocity modulation is applied using an additional coil operable to modify the horizontal deflection of the beam. The additional coil is driven with edge transition information extracted from the luminance video signal by a driver circuit.
The SVM coil may be a flat wound laminated coil, which in some earlier SVM applications was mounted on the purity/static convergence magnet holder, to affect the electron beams after they exit the focus (G4) grid of the electron gun. Printed circuit coils are also known and these may be positioned under the static convergence magnets or under the deflection yoke.
SVM coil placement in the immediate proximity of the electron guns reduces the SVM coil field strength applied to the electron beam. The electron guns normally include permeable ferromagnetic material (e.g., steel) which tends to confine the SVM field to the permeable material. The electron guns also include non-ferromagnetic conductive material which dissipates the SVM field by eddy current induction. In short, the ferrous and conductive material of the electron guns tend to partially dissipate the SVM field and shield the electron beams from interaction therewith.
It is possible to combat adverse effects of shielding and dissipation inherent in a particular SVM coil mounting location by, for example, increasing the SVM drive current, however this may require improved performance current drivers, with increased device dissipation. The number of coil turns may be increased to achieve the desired SVM deflection sensitivity, however, this increase in inductance may result in a reduction in the high frequency performance of the SVM coil to a frequency below that required by the spectral composition of the displayed signal. SVM deflection sensitivity may be improved by positioning the SVM coil as close as possible to the electron beam, since magnetic field strength decreases in proportional the inverse of the spacing distance raised to a power. However, positioning, for example, on the tube neck may result in mechanical mounting problems of the other neck components which may be placed to overlap such an SVM coil position. Thus any SVM coil positioned in contact with a tube surface should have a minimized cross sectional dimension for both the coil and the connections thereto.